Conveying devices, such as those which are used for moving objects between different stations in a factory, usually comprise a conveying track in the form of a belt or a chain. The conveying tracks can be recessed in a trench with vertical side surfaces. Alternatively, they can be located on the horizontal upper surfaces of the trench or arranged in some other way. The objects to be conveyed are arranged slidably in relation to the conveying tracks, either directly or via carrying means, which are also known as pallets. A specific type of pallet is a puck, which is a round carrier adapted for smaller and more lightweight objects.
A pallet is conveyed along the conveying device, which comprises different work stations. At a work station, the pallet will be stopped such that the operation may be performed on the transported object. In some cases, the object may pass a specific work station without an operation being performed on the object.
If the conveying device is of a conventional type, in which all work stations are positioned after each other and the operations on the objects are performed synchronous, every pallet will stop at each work station. If no operation is to be performed on an object at a specific work station, the object will have to wait at that work station until the operation on the previous object at that work station is ready, until the object can continue. All pallets are in this case released at the same time and moves at the same time. One advantage of such a system is that it is easy to predict the performance of the system. One disadvantage of such a system is that it is not very flexible. If the operation time at a work station is long, all objects will have to wait that time, even if there is no operation performed on all objects. Thus, such systems are mostly used for a single product or when the differences in the products are small.
In other systems, the operations on the objects are performed asynchronous, i.e. the pallet stops at a work station where an operation is to be performed, and is released when the operation is ready. In such systems, a relatively large buffer is required between each work station in order to compensate for the different through-flow times of the products and for the different operation times at a work station.
Another way of compensating for different operation times at a work station is to use two or more work stations that perform the same task. They are often positioned close to each other, and the object is stopped at the work station that is free. In this way, the through-flow time of the system can be balanced in order to avoid bottlenecks in the system. In such a system, the pallet may either be stopped at the work station or may pass the work station without an operation being performed.
In very simple systems, it may be enough to stop the pallet on the conveyor device and hold it there with a simple stop mechanism. This is enough if a simple operation such as loading or unloading is to be performed. For more advanced operations, the pallet and/or work piece is normally deviated from the main conveyor track into a position beside the conveyor track. In this way, the operation can be performed on the work piece without influence from the conveyor track, which continues to move.
The work station may be positioned at a sub-conveyor located on the side of the main conveyor. Such a solution will require extra space, an extra sub-conveyor, and means to divert objects to the sub-conveyor. This solution will work well in some cases, but is rather inflexible and complicated for smaller operations.
When the pallet is stopped at the work station, it is important that the pallet is held in a specific, predefined position in order for the work station to perform the operation. If the locating accuracy of the work station is not good enough, the work piece may be damaged or the operation may not be as good as expected. In order to obtain a repeatable positioning of the pallet and/or the work piece, the working station is normally equipped with a specific fixating means that holds the work piece during the operation.
Such fixating means are often provided with a number of sensors in order to establish the position of the pallet and/or the work piece before it reaches the stop and specific holding means operated in dependency of the sensors in order to hold the work piece. Such a solution may in some cases be more complicated than necessary. There is thus room for improvements.